Most prior art "digital" radiography techniques do not provide truly digital images. The quotes are added here to indicate that most prior art techniques are basically analog and integrative in nature, with digitization occurring at the last step. A first prior art radiography technique uses a phosphor x-ray detector with an image intensifier, followed by very fast optics and a television camera and is referred to as (PIITV). A second prior art radiography technique is "photostimulable phosphor computer radiography" (PPCR). However, both of these prior art radiography techniques suffer from severe drawbacks.
In the PIITV radiography technique, intensifier devices are required. These image intensifier devices are as much as 30-40 cm in diameter and are very expensive. Additionally, the TV cameras used in the PIITV technique are not capable of resolutions greater than approximately 1000 lines. Furthermore, the image intensifier/TV system has intrinsic noise that degrades image quality at low doses.
In the PPCR radiography approach a storage phosphor screen is used. Minute crystals of, for example, BaFX:Eu.sup.2+, where X=Cl, Br, I, in a thin layer absorb the radiation and generate a latent image in trapped energy states. Image development is accomplished by scanning the exposed plate with an infrared laser beam (He-Ne), producing photostimulated luminescence in the UV range. The laser scans the exposed plate with a spot having a 100 .mu.m diameter size. The UV light is collected with a light guide and detected with a high-sensitivity photomultiplier tube. Digitization of the photomultiplier signal is followed by extensive processing to produce the image. The scan takes a rather long time, on the order of a minute or more, which imposes a limitation in some circumstances.
Additionally, PPCR has a very high dynamic range in principle, but has been limited in practice to about 10 bits. The largest image size is about 2048 pixels. Nevertheless, the PPCR approach, as with the PIITV approach, does not fall into the category of true digital radiography.
Recently methods, such as those disclosed in U.S. Pat. No. 4,937,453 to Nelson, have been proposed to take advantage of advances in semiconductor technology. The Nelson reference discusses a variety of stacking, edge-on, and drift-device configurations that serve to increase the x-ray stopping power of semiconductors. Stopping power is an important issue for a low Z material such as silicon. However, the Nelson reference does not address the issue of digital versus analog signal processing.
True digital radiography produces an immediate digital signal based upon the interaction of an x-ray with a detector. That is, no intermediate steps such as the laser scanning of a phosphor screen are required to achieve a digital signal. Although such methods provide certain improvements over "non-digital" radiography techniques, the methods cited in the Nelson reference only detect the interaction between an x-ray and the material which forms the detector on which the x-ray is incident. In so doing, the Nelson device does not quantify or extract beneficial information contained in each interacting x-ray. Specifically, the differing energies of each of the interacting x-rays are not quantified by the Nelson reference.
Thus, many prior art radiography techniques do not provide true digital x-ray detection. Additionally, prior art methods which do provide true digital x-ray detection, are unable to extract valuable considerable energy information contained within each interacting x-ray.
Consequently, a need exists for radiography technique which is truly digital, and which is able to extract and quantify valuable energy information contained within each interacting x-ray.